Can I Mix 20W40 And 15W40 Oil – A Practical Guide For Emergencies

ByRobert Lozano

You can mix 20W-40 and 15W-40 oil in an emergency or for a small top-off. Both oils share the same viscosity (40) at operating temperature, ensuring protection when the engine is hot. However, the mixture will have altered cold-start properties, so it’s best to perform a full oil change with the correct grade as soon as possible.

You’re halfway through a long road trip or deep on a favorite trail, and the dreaded oil light flickers on. You pull over, pop the hood, and the dipstick confirms it—you’re low. You dig through your kit and find a spare quart, but it’s 15W-40, and your engine runs on 20W-40. It’s a classic DIY dilemma that can cause a lot of stress.

We get it. The fear of causing engine damage is real, and the numbers on oil bottles can seem like a cryptic code. You need a clear, straightforward answer from someone who’s been there, without all the confusing technical jargon.

This guide will give you that answer. We’ll break down exactly what those numbers mean, explain the real-world consequences of mixing these two specific oil weights, and provide a complete can i mix 20w40 and 15w40 oil guide so you can make a safe, confident decision, whether you’re in your garage or on the side of the road.

Decoding the Numbers: What Do 20W-40 and 15W-40 Really Mean?

Before we can talk about mixing, we need to understand what we’re working with. Those numbers on the bottle aren’t random; they represent the oil’s viscosity, which is just a fancy word for its thickness or resistance to flow.

The Society of Automotive Engineers (SAE) sets these grades, and for multi-grade oils like these, there are two numbers to consider.

The “W” Number: Your Engine’s Cold-Start Protector

The first number, followed by a “W,” stands for “Winter.” This rating tells you how well the oil flows when it’s cold, like during a frosty morning startup.

A lower “W” number means the oil is thinner and flows more easily at low temperatures. In our case:

  • 15W oil flows better in the cold than 20W oil.
  • 20W oil is slightly thicker at cold temperatures.

This is the most important difference between these two oils. Proper flow on a cold start is crucial for getting lubrication to vital engine parts quickly and preventing wear.

The “40” Number: High-Temperature Performance

The second number—the “40” in both 20W-40 and 15W-40—indicates the oil’s viscosity when the engine is at its normal operating temperature (around 212°F or 100°C).

Because both oils are rated as a 40 weight at this temperature, they provide a virtually identical level of protection once your engine is warmed up and you’re cruising down the highway or climbing a trail. This shared characteristic is why mixing them in a pinch is even possible.

Additive Packages: The Unseen Difference

Beyond viscosity, motor oil is a complex blend of base oils and chemical additives. These additives include detergents to keep the engine clean, anti-wear agents, rust inhibitors, and more.

Different brands use their own proprietary formulas. While modern oils are designed to be compatible, mixing brands means you’re also mixing these carefully balanced additive packages, which could slightly reduce their overall effectiveness.

So, Can I Mix 20W-40 and 15W-40 Oil? The Short and Long Answer

Here’s the bottom line you came for. The answer is a qualified “yes.” It’s not ideal, but it’s far better than the alternative of running your engine with dangerously low oil.

The “Yes, in an Emergency” Scenario

If your oil level is low and you need to add a quart to safely get to your destination or the nearest auto parts store, absolutely mix them.

An engine with slightly the wrong oil viscosity is infinitely better than an engine with not enough oil. Lack of oil leads to friction, overheating, and catastrophic failure. A small top-off with a closely related oil grade will prevent that.

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Why It’s Not a Long-Term Solution

Mixing these oils should only be a temporary fix. When you combine 20W-40 and 15W-40, the resulting blend will have a cold-start viscosity somewhere between 15W and 20W.

This might not be a big deal in a warm climate, but in colder weather, this altered viscosity could slightly slow down lubrication on startup. To ensure your engine gets the exact protection it was designed for, you should plan on a full oil and filter change soon. This is one of the most important can i mix 20w40 and 15w40 oil best practices.

The Golden Rule: Same Brand, Same Type

If you have a choice, it’s always best to mix oils of the same type (conventional with conventional, synthetic with synthetic) and, if possible, the same brand. This minimizes the chances of any issues with additive compatibility.

However, in a true emergency, any API-certified oil of a similar grade is better than nothing.

Common Problems with Mixing 20W-40 and 15W-40 Oil

While mixing is generally safe for a short-term top-off, it’s helpful to be aware of the potential, albeit minor, issues. Understanding these common problems will reinforce why it’s a temporary measure.

Compromised Cold-Start Protection

This is the primary concern. If your vehicle specifies 15W-40 for cold weather performance and you add 20W-40, you’ve made the oil slightly thicker on startup. Conversely, adding 15W-40 to a 20W-40 system thins it out.

While a single quart in a five-quart system won’t cause a major change, it technically moves the oil’s performance outside the manufacturer’s exact specifications.

Slight Dilution of Additives

Think of an oil’s additive package like a specific recipe. When you pour in a different “recipe,” you change the final dish. The oil will still work, but the specialized detergents or anti-wear agents might not perform at 100% of their designed capacity.

Voiding Your Warranty (A Remote Possibility)

For vehicles still under warranty, this is something to consider. Manufacturers specify an exact oil grade in the owner’s manual. If an engine failure occurs and an oil analysis shows a mixed or incorrect viscosity, the dealer could potentially deny the warranty claim.

This is highly unlikely for a simple top-off, but it’s a key reason to return to the recommended oil as soon as possible.

A Practical Guide: How to Mix 20W-40 and 15W-40 Oil Safely

If you find yourself in this situation, don’t panic. Follow this simple how to can i mix 20w40 and 15w40 oil process to handle it like a pro. This care guide ensures you do it right.

  1. Assess the Situation: Is it a True Emergency?

    First, confirm the oil is actually low. Park on a level surface. If the oil light came on while driving, let the engine cool for at least 10-15 minutes to allow the oil to drain back into the pan for an accurate reading.

  2. Check Your Oil Level Correctly

    Pull out the engine oil dipstick, wipe it clean with a rag or paper towel, fully re-insert it, and then pull it out again. Check the level against the markings. If it’s at or below the “ADD” line, you need to top it off.

  3. Top-Off with the Minimum Amount Necessary

    Unscrew the oil filler cap (it usually has an oil can symbol). Using a funnel to prevent spills, add about half a quart of the mismatched oil. Wait a minute, then re-check the dipstick. Add more only if needed to bring the level into the safe zone (between “ADD” and “FULL”). Do not overfill!

  4. Plan for a Proper Oil Change

    You’ve averted disaster. Now, make a plan. At your earliest convenience, perform a complete oil change with a new filter and the manufacturer-recommended oil grade. This resets everything and ensures your engine has the best possible protection moving forward.

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Sustainable and Eco-Friendly Oil Practices

Being a responsible vehicle owner goes beyond just mechanical maintenance. Incorporating sustainable practices into your routine is easier than you think and makes a real difference.

Reducing Waste by Topping Off

One of the hidden benefits of can i mix 20w40 and 15w40 oil in an emergency is waste prevention. By saving your engine from catastrophic failure, you prevent the massive environmental and resource cost of replacing it. A small top-off is a very eco-friendly can i mix 20w40 and 15w40 oil choice compared to the alternative.

Proper Disposal is Non-Negotiable

When you do your follow-up oil change, proper disposal of the old oil is critical. Never, ever dump used motor oil down a drain, on the ground, or in the trash. A single quart can contaminate thousands of gallons of water.

Pour the old oil into an old oil jug or a dedicated drain pan. Take it to any major auto parts store (like AutoZone or O’Reilly) or your local municipal recycling center. They will accept it for free and ensure it gets recycled properly.

Frequently Asked Questions About Mixing Motor Oils

What happens if I mix synthetic 15W-40 with conventional 20W-40?

It is safe to mix synthetic and conventional oils. However, the mixture will lose the high-performance benefits and extended life of the full synthetic oil. The blend will essentially perform like a conventional or synthetic-blend oil.

Is it better to run low on oil or top off with the wrong weight?

Always top off. Running an engine low on oil is one of the fastest ways to destroy it. Insufficient oil pressure will cause metal-on-metal contact, extreme heat, and seizure of critical components. The minor risks of a slightly off-spec viscosity are nothing compared to the certainty of damage from oil starvation.

How much can I mix?

For an emergency top-off, a quart or less in a typical 5-6 quart passenger car or light truck system is a reasonable guideline. If you need to add more than that, you should be prioritizing an immediate oil change and checking for potential leaks.

Does this advice apply to both diesel and gasoline engines?

Yes, the principles of viscosity mixing are the same. However, you must ensure both oils meet the required service classification for your engine type. For example, modern diesel engines often require API “CK-4” rated oil, while modern gasoline engines use “API SP.” Check the “donut” symbol on the back of the bottle to confirm.

Mixing 20W-40 and 15W-40 isn’t something you should do every day, but knowing you can do it safely in a tight spot is empowering. The key is to see it as a temporary bridge to a proper solution.

By understanding what the numbers mean and following a few simple steps, you can handle a low-oil situation with confidence, protect your engine, and get back on your way. Keep up with your regular maintenance, and always try to carry a spare quart of the correct oil for true peace of mind.

Keep those wrenches turning, stay safe on the road and trail, and give your engine the care it deserves!

Robert Lozano
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